1. Field of the Invention
The present invention relates to spill valves, i.e., tank overflow valves which relieve liquid from tanks in an overfill condition when the pressure exceeds a predetermined limit, thereby to protect against rupture of the tank. The invention has particular application to marine liquid cargo tanks.
2. Description of the Prior Art
Industry standards, government regulations and international conventions require certain safety features in liquid cargo tanks, including marine cargo tanks. One such common safety feature is the provision of overflow or spill valves. Such valves are designed to protect against rupture or other damage to the tank in the event of an overfill condition. Typically, liquid is introduced to the tank under pressure exerted by pumps or the like during the filling operation. If the filling operation is not terminated at or before the time the tank is filled, liquid pressure may build up in the tank and eventually cause it to rupture. Marine cargo tanks, such as in barges or the like, may rupture at superatmospheric pressures in the range of from about 1 psig to about 3 psig. A spill valve is biased closed with a predetermined force such that the valve will open at a predetermined liquid pressure in the tank which is less than the pressure at which the tank will rupture, thereby relieving liquid from the tank and preventing further pressure buildup.
Prior spill valves are typically of the poppet type and include a valve plate which is biased to a closed position against a seat. Heretofore, spill valve designs have typically prescribed a valve travel between closed and fully open positions through a distance which is approximately one-quarter the diameter of the port opening which is controlled by the valve. Thus, the maximum flow rate through the open valve is limited. The junction between the port nozzle and the tank wall has typically been a right angle, which results in a flow coefficient of approximately 0.68, which further serves to limit flow through the valve. The maximum flow rate through the spill valve, in turn, limits the filling rate of the tank since if the filling rate exceeds the maximum flow rate through the spill valve, pressure will continue to build up in the tank even after the spill valve is fully opened.
In prior poppet-type spill valves, when the valve first starts to open the liquid will exit laterally around the perimeter of the poppet through a very narrow annular opening. The flow through this reduced opening produces a Venturi effect with increased flow rate an locally reduced pressure. This tends to reduce the overall lifting force on the poppet, requiring a correspondingly higher tank pressure in order to continue to move the poppet in an opening direction.
Furthermore, it has been found that in prior poppet-type spill valve designs the valve member tends to oscillate between open and closed positions once it has been opened, i.e., it will fluctuate between the fully open and closed conditions, sometimes with such force that it can deflect the tank wall. This not only impairs flow through the valve, but also can result in damage to the valve or to the tank wall.
Typically, the spill valve has an elastomeric seal element disposed for engagement with the valve member and the seat when the valve is closed to provide a fluid-tight seal. In marine applications the seal region is subject to damage and degradation from weather, such as boarding seas, from debris, such as sand and the like, and from icing buildup conditions. This may necessitate replacement of the elastomeric seal element. Typically, the valve member is spring-biased to its closed position and the valve construction is such that in order to permit cleaning or servicing of the valve structure and/or repair or replacement of the seal element, it is necessary for the valve to be disassembled.
Another drawback of prior spill valve designs is that, when the valve is open, the liquid lading which is exiting the valve under pressure, is squirted out either upwardly, much like a geyser, or laterally to the side. In either event, it may spray a considerable distance and constitute a hazard for tankermen in the vicinity.